Figure 6.

A mutant stabilizing the loop–loop interaction in the free form of the adenine-sensing riboswitch RNA. Overlay of the imino region of a ¹H,¹⁵N-HSQC spectrum of the free form of the wild-type adenine-sensing riboswitch RNA (black) and a A30G/U40C double mutant of the adenine-sensing riboswitch RNA (red) both in the absence of Mg²⁺. Signals for the imino groups of G37 and U34 are barely visible in the wild-type RNA but are readily detected in the mutant RNA. Only two signals for the uridine imino groups in an U:U base pair are observable in the mutant. In addition, the imino signals for the wobble G:U base pair (arrows) are absent in the mutant. Inset: Secondary structure of the A30G/U40C double mutant. The mutated base pair at the apical tip of helix II is shaded in red.